A spectroscopic analyzer such as an FTIR analyzes sample components on the basis of a wavenumber intensity distribution (optical spectrum) of light beams passing through a sample. In order to detect the intensity of the light beams passing through the sample, a photodetector such as a MCT photodetector is used. However, it is of course required to obtain the relationship (calibration curve) between the output value of the photodetector and the intensity of the incident light beams before analyzing.
Therefore, conventionally, a calibration curve has been prepared by making predetermined reference light beams enter a photodetector to obtain the output value of the photodetector.